1. Field of the Invention
This invention relates to tracking the position of one or more untethered objects.
2. Related Art
The ability to track the location and/or orientation of an object (sometimes referred to herein either singly or collectively as the xe2x80x9cpositionxe2x80x9d of the object) can be desirable for a variety of purposes. A number of different position tracking systems exist. The operation of such position tracking systems is typically tailored in accordance with the characteristics of the particular application or applications for which the system is intended to be used, thus producing a position tracking system having particular characteristics.
Position tracking systems can be xe2x80x9ctetheredxe2x80x9d or xe2x80x9cuntethered.xe2x80x9d In a tethered position tracking system, the object being tracked is either mounted on, attached to, or connected by another physical object (such as a wire) to a part of the tracking apparatus that is fixed with respect to the space (xe2x80x9ctracking spacexe2x80x9d) within which the position of the object is to be tracked. In an untethered position tracking system, there is no such physical mounting, attachment or connection. Typically, a tethered position tracking system restricts the movement of the tracked object with respect to the tracking space more than does an untethered system. Therefore, a tethered position tracking system may be undesirable or unusable for applications in which unfettered movement of the object is desired or necessary, such as applications in which the position of an object is to be tracked within a relatively large scale tracking space. Untethered position tracking systems, on the other hand, may not be as accurate as tethered position tracking systems, as a consequence of the less direct connection between the object being tracked and the tracking apparatus. Thus, untethered position tracking systems may be undesirable or unusable for applications in which a high degree of accuracy is required. Such applications tend to be, in general, applications in which the position of the object is to be tracked within a relatively small tracking space.
For example, the Global Positioning System (GPS), a satellite navigation system developed by the United States Department of Defense, is an untethered position tracking system used to track objects on a global (i.e., very large) scale. Multiple transmitters are positioned at fixed and known positions relative to the Earth. The transmitters emit signals that can be received by a receiver that is typically positioned on or near the surface of the Earth. The receiver receives signals from multiple transmitters. The duration of time required for each signal to travel from the corresponding transmitter to the receiver can be ascertained and used to determine the position of the receiver.
Because GPS transmitters must produce a signal that is strong enough to be detected globally, GPS transmitters require a relatively large amount of power. It is often not feasible, therefore, to implement a GPS transmitter having sufficient power that is small enough to be constructed as part of an object to be tracked. Consequently, as described above, a GPS tracking system is implemented so that the receiver, rather than the transmitter, is constructed as part of an object to be tracked. Further, the size of a physical device that embodies a receiver typically cannot be reduced beyond a certain point (i.e., there must be a minimum area devoted to receiving the incoming signal), even for very low power position tracking signals. Thus, a GPS tracking system is generally not well-suited to tracking the position of small objects.
Depending upon the characteristics of the hardware used in, and the geometry of, a particular implementation, a GPS tracking system can provide an estimate of the location of an object within approximately 20-30 meters in each direction. Thus, a GPS tracking system is useful in tracking an object within a relatively large scale tracking space (e.g., tracking the global position of an object), where such accuracy is acceptable.
A Polhemus tracking system is an example of a position tracking system that can be used for position-tracking in small tracking spaces. A Polhemus tracking system makes use of magnetic sensing to enable tracking. In a Polhemus tracking system, a magnetic field generator (transmitter) creates a magnetic field in the tracking space. A magnetic sensor (receiver) that is attached to an object being tracked includes a set of coils that are positioned at a known location and orientation with respect to the object. The magnetic field induces a current in the coils. The magnetic sensor is connected to a computer which can ascertain the magnitude and direction of the currents induced in the coils, and determine the location and orientation of the magnetic sensor (and, thus, the object) from those currents. The sensing electronics and the required computational capacity are each sufficiently complex that the magnetic sensor and computer can not typically be constructed together in an apparatus that is sufficiently small to be attached to the (typically small) object being tracked. Thus, Polhemus tracking systems are constructed as tethered systems in which the magnetic sensor is connected to the computer by a wire.
Similar to a GPS tracking system, the transmitter (magnetic field generator) of a Polhemus tracking system requires a relatively large amount of power, necessitating that the transmitter be embodied by a relatively large physical device. Since an object to be tracked is typically small, it is generally not feasible or desirable to implement a Polhemus tracking system so that a transmitter is attached to the object, rather than a receiver. Further, as indicated above, the size of a physical device that embodies a receiver typically cannot be reduced beyond a certain point. Thus, a Polhemus tracking system may not be capable of use in tracking the position of very small objects.
A Polhemus tracking system can enable accurate determination of the position of an object. However, since the object is tethered to the computer which performs the computations necessary to determine the position, a Polhemus tracking system can track the position of an object only within a relatively small tracking space. Further, since the objects are tethered, the number of objects that can feasibly be tracked is rather small, since, as the number of objects increases, the tethering wires become increasingly likely to become entangled.
Another example of a small scale position tracking system is a computer mouse that includes three receivers, fixed in position at known locations external to the mouse, that are adapted to receive ultrasound signals emitted by a transmitter that is attached to the mouse. The location of the transmitter (and, thus, the mouse) at the time a signal is emitted can be determined from the duration of time required for the signal to reach each receiver. The mouse is tethered to a computer to enable power to be supplied to the mouse, and to enable communication between the mouse and a computer with which the mouse is used.
This position tracking system also suffers from some limitations. The system is only adapted to track a single object. The system is also only adapted for use in tracking an object within a very small tracking space, e.g., a range of about a yard from the receivers. Finally, the object is tethered, thus limiting the range of motion of the object.
There are a variety of situations in which it is desirable to track the position of an object that is part of a computer interface. Such objects can be referred to as xe2x80x9cactive objectsxe2x80x9d and that term is sometimes used herein for that purpose. Further, herein, xe2x80x9ccomputer interfacexe2x80x9d is used broadly to describe any point of interaction between a system including a computational device and a user of that system. For example, a glove worn by a user of a virtual reality system can be an active object. Or, a set of computer-controlled toy vehicles can be active objects. The chess pieces in a computerized chess game can be active objects.
The position tracking systems described above each have inadequacies when used to track one or more objects. The GPS tracking system is typically inappropriate for tracking active objects, since the scale of the tracking space is usually much smaller than the scale of a tracking space for which a GPS tracking system is appropriate. Further, the active objects themselves may be too small to enable an appropriate GPS receiver to be attached thereto. While a Polhemus tracking system is adapted to track objects in a tracking space having a scale that is more appropriate for tracking active objects, a Polhemus tracking system may also not be able to track very small active objects. Further, the tethering of a Polhemus tracking system may undesirably limit the range of motion of the active objects or the number of active objects that can be tracked, or otherwise undesirably interfere with the operation or use of the active objects. Finally, the computer mouse tracking system can track only a single active object. The size of the tracking space is also severely limited. Further, the tethering of the active object limits the range of motion of the object.
The invention can enable the position (i.e., location and/or orientation) of one or more untethered objects to be tracked. One or more transmitters, each of which has a fixed and known positional relationship to a corresponding object, each emit a sequence of position tracking signals. Each transmitter can emit a unique, identifiable position tracking signal so that the transmitter (and the object to which the transmitter is coupled) from which a signal was emitted can be identified. When a transmitter is coupled to each of multiple objects, the invention can enable the position of multiple objects to be tracked. When multiple transmitters are coupled to a single object, the invention enables both the location and the orientation of the object to be tracked. Multiple receivers receive the position tracking signals and record the times (time-stamp) at which the position tracking signals are received. (The receipt of a position tracking signal is termed a xe2x80x9cdetection event.xe2x80x9d) When multiple transmitters are being used, the receivers are adapted to enable identification of the transmitter from which a position tracking signal was emitted. The recorded times can be used to determine the location (and, in some cases, the orientation) of each object at a time that a position tracking signal was emitted from the corresponding transmitter. Depending upon the number and relative locations of the receivers used, the invention can be used to track the position of an object or objects in one, two or three dimensions.
The position tracking signals can be of any type that enables the transmitter and object to be untethered, such as acoustic. (e.g., ultrasonic) or electromagnetic (e.g., microwave) signals. Since the objects are untethered, a system according to the invention can be used to track more objects, and track the objects as they move in a larger space, than is possible with the tethered tracking systems discussed above. The position tracking signals can advantageously be signals requiring a relatively small amount of power to generate (e.g., ultrasonic signals or radiofrequency signals), thus enabling the transmitter to be embodied in a physically small device, enabling the transmitter to be physically coupled to relatively small objects; in particular, small, low power transmitters can be used that enable tracking of objects that are smaller than those that can be tracked by the GPS or Polhemus tracking systems discussed above. Depending upon the transmitter signal power used, the invention can be used to track objects in small and medium-scale tracking spaces. In particular, the use of ultrasonic signals can be advantageous because ultrasonic signals are relatively computationally inexpensive to process.
In one embodiment of the invention, a system for tracking the position of an object includes a transmitter and at least four receivers. The transmitter is coupled to the object as described above. The receivers are positioned at fixed and known positions relative to a tracking space (which can itself be moving). As described above, the receivers can time-stamp detection events, which can then be used to determine the location of the transmitter (and, therefore, the object) at times that a signal was emitted by the transmitter. Preferably, the positions of the receivers are not coplanar, so that the system can track the location of the object in three dimensions. Additionally, preferably, the position tracking signals are ultrasonic, radiofrequency or other signals that require a relatively small amount of power to transmit, so that the transmitter can be embodied by a relatively small (e.g., 0.1 to 2 cubic inch) device. As can be appreciated, a system in accordance with this embodiment of the invention can advantageously enable the location of a small untethered object to be tracked in three dimensions. The system can be particularly useful for tracking the location of an object in a small or medium scale tracking space.
In another embodiment of the invention, a system for tracking the position of multiple objects includes multiple transmitters and multiple receivers. Each of the transmitters is coupled to a corresponding one of the objects as described above. As in the above embodiment, the receivers are positioned at fixed and known positions relative to a tracking space (which can itself be moving). The transmitters are implemented so that a position tracking signal emitted by a particular transmitter can be identified as having been emitted by that transmitter. As described above, the receivers can time-stamp detection events. The receivers can also identify the transmitter associated with a detection event (i.e., the transmitter that emitted a signal detected by the receiver). The time-stamped detection events associated with a particular transmitter can be used to determine the location of that transmitter (and, therefore, the object corresponding to that transmitter) at times that a signal was emitted by that transmitter. Depending upon the number and positioning of the receivers used, such a system can track the locations of the objects in one, two or three dimensions. As in the embodiment above, preferably, the position tracking signals are ultrasonic, radiofrequency or other signals that require a relatively small amount of power to transmit, so that the transmitter can be embodied by a relatively small device. A system in accordance with this embodiment of the invention can advantageously enable the location of multiple untethered objects to be tracked. In particular, the system can be useful for tracking the locations of small objects in a small or medium scale tracking space.
In yet another embodiment of the invention, a system for tracking the position of an object includes at least one pair of transmitters and multiple receivers. The pair of transmitters are coupled to the object as described above. As in the above embodiments, the receivers are positioned at fixed and known positions relative to a tracking space (which can itself be moving). The transmitters are implemented so that a position tracking signal emitted by a particular transmitter can be identified as having been emitted by that transmitter. Further, the transmitters can include synchronized clocks so that position tracking signals can be emitted from a pair of transmitters at the same time. (If the rotational rate of an object is limited to be, or is known to be, relatively small, position tracking signals need not be emitted at the same time from transmitters coupled to that object and, consequently, those transmitters need not include synchronized clocks.) As described above, the receivers can time-stamp detection events, as well as identify the transmitter associated with a detection event. The time-stamped detection events associated with a particular transmitter can be used to determine the location of that transmitter (and, therefore, the object corresponding to that transmitter) at times that a signal was emitted by that transmitter. Further, the locations at a particular time of a pair of transmitters coupled to an object can be used to determine the orientation of that object at that time. A system in accordance with this embodiment of the invention can advantageously be used to track both the location and the orientation of one or more untethered objects. Further, depending upon the number and positioning of the receivers used, such a system can track the location and orientation of the object(s) in one, two or three dimensions. As in the embodiments above, the position tracking signals can be signals that require a relatively small amount of power to transmit, thereby enabling use of a small transmitting device so that the location and orientation of one or more small objects can be tracked in a small or medium scale tracking space.
In still another embodiment of the invention, a system for tracking the position of an object includes an inertial device, a transmitter, multiple receivers and a controller. The transmitter is coupled to the object as described above. The inertial device is also coupled to the object. Again, as in the embodiments above, the receivers are positioned at fixed and known positions relative to a tracking space (which can itself be moving). The inertial device obtains inertial tracking information and represents a first position tracking subsystem. The inertial tracking information obtained by the inertial device can be communicated to the transmitter so that the information can be combined with the position tracking signal. As described above, the location of the transmitter (and, therefore, the object) at times that a signal was emitted by the transmitter can be determined by the controller from time-stamped detection events recorded by the receivers. Operating in this way, the transmitter and receiver represent a second position tracking subsystem. The inertial tracking information can be used to refine or augment the location information obtained by the transmitter/receiver position tracking subsystem. This can occur in one or both of two ways. First, the inertial device can typically obtain inertial tracking information more quickly than the transmitter/receiver position tracking subsystem can obtain location information, thereby enabling direct determination of the location of the object at times intermediate those at which the transmitter/receiver position tracking subsystem determines the location of the object. Second, the inertial device enables acceleration information to be determined, so that, even if the inertial tracking information is not obtained more quickly than the transmitter/receiver position tracking subsystem obtains location information, the acceleration information enables the location of the object at such intermediate times to be determined. Further, the location information determined by the transmitter/receiver position tracking subsystem is, over time, more reliable than location information determined by the inertial device, which tends to drift. Thus, the location information determined by the transmitter/receiver position tracking subsystem can be communicated by the controller to the inertial device to periodically recalibrate the inertial device, correcting any drift that may have occurred. As can be appreciated, a system according to this embodiment of the invention exploits the relative strengths of the two position tracking subsystems to result in an overall system that can track the location of an object more accurately than can either of the subsystems operating alone.
In another embodiment of the invention, a system for tracking the position of multiple objects includes multiple transmitters and multiple receivers. Each of the transmitters is coupled to a corresponding one of the objects as described above. Additionally, each of the receivers is coupled to a corresponding one of the objects so that a transmitter/receiver pair is coupled to each object. The transmitters are implemented so that a position tracking signal emitted by a particular transmitter can be identified as having been emitted by that transmitter. Further, the transmitters include synchronized clocks so that position tracking signals can be emitted from the transmitters at the same time. As described above, the receivers can time-stamp detection events, as well as identify the transmitter associated with a detection event. The time-stamped detection events from each receiver are communicated to one or more controllers, each of which can use the time-stamped detection events to determine the relative locations of the transmitters (and, therefore, the objects corresponding to the transmitters) at times that position tracking signals were emitted by the transmitters. In a further particular embodiment, a pair of transmitters are coupled to each object so that the orientation of the objects can also be determined. In another particular embodiment, an inertial device is coupled to each object so that inertial tracking information can be obtained for the object, thereby enabling improved location information to be obtained for the objects. A system in accordance with this embodiment of the invention can, like other embodiments above, advantageously enable the position of multiple untethered objects to be tracked. As in the embodiments above, the position tracking signals can advantageously be signals that require a relatively small amount of power to transmit, thereby enabling use of a small transmitting device so that the location and/or orientation of small objects can be tracked in a small or medium scale tracking space. As in some of the embodiments above, depending upon the number of receiver/transmitter pairs, a system according to this embodiment of the invention can track the locations of the objects in one, two or three dimensions.
The invention can be used for any application in which it is desired to track the position of one or more objects in a small or medium-scale space. For example, the invention can be used to track the location of mobile equipment in an industrial environment, such as a factory. Or, the invention can be used to track the location and orientation of each of a group of remotely operated cameras. The invention can also advantageously be used to track active objects (i.e., objects that are part of a computer interface).